The present invention relates to an NOx sensor control circuit unit and an NOx sensor system including the control circuit unit.
NOx-concentration measuring apparatus using an NOx sensor for detecting the concentration of nitrogen oxides (hereinafter called NOx) in exhaust gases from internal combustion engines and the like are disclosed, for example, in European Patent Application Laid-Open No. 0678740A1 and SAE Paper No. 960334, pp. 137-142, 1996. An NOx sensor used in such a conventional NOx-concentration measuring apparatus is composed of oxygen-ion conductive solid electrolyte layers that form a first processing chamber and a second processing chamber. The first processing chamber communicates with the gas to be measured (hereinafter called xe2x80x9ca measurement gasxe2x80x9d) via a first diffusion-controlling passage, and the second processing chamber communicates with the first processing chamber via a second diffusion-controlling passage. The solid electrolyte layer of the first processing chamber is sandwiched between porous electrodes so as to form a first pump element and an oxygen-concentration-measuring cell. The solid electrolyte layer of the second processing chamber is sandwiched between porous electrodes so as to form a second pump element.
In the thus-configured NOx-concentration measuring apparatus, a current is made to flow through the first pump element such that an output voltage from the oxygen-concentration-measuring cell attains a predetermined value, thereby controlling the concentration of oxygen in the first processing chamber to a constant level. At the same time, a constant voltage is applied to the second pump element to thereby pump out oxygen from the second processing chamber. As a result, the NOx concentration of a measurement gas can be obtained from the current flowing through the second pump element.
A measurement gas, e.g., exhaust from an internal combustion engine or the like, contains gas components other than NOx, such as oxygen, carbon monoxide and carbon dioxide. Thus, in the aforementioned NOx-concentration measuring apparatus, first, the oxygen concentration of the first processing chamber is controlled to a very low level by means of the first pump element. Then, in the second processing chamber which receives the measurement gas controlled to a low oxygen concentration, a constant voltage is applied to the second pump element in a direction such that oxygen is pumped out from the second processing chamber. As a result, NOx contained in the measurement gas is decomposed into nitrogen and oxygen by means of the catalyzing function of the porous electrodes of the second pump element, and the thus-generated oxygen is then pumped out from the second processing chamber. Thus, the NOx concentration of the measurement gas can be obtained from the current flowing through the second pump element without influence of other gas components contained in the measurement gas.
In the NOx-concentration measuring apparatus, in order to accurately detect the NOx concentration, the NOx sensor must be heated to a predetermined activation temperature (for example, about 850xc2x0 C.) so that the cells are activated. Therefore, the apparatus is provided with a heater for heating the NOx sensors.
On the other hand, in a lean burn engine, for example, which is run at a lean air-fuel ratio (namely, a high ratio of air to fuel), the resulting exhaust gas tends to contain a relatively large amount of NOx. Accordingly, in order to suppress NOx emissions, the above-mentioned NOx-concentration measuring apparatus is often used to monitor the state of a reducing catalyst installed in an exhaust line. Specifically, the NOx sensor is installed in an exhaust passage of an internal combustion engine downstream of a reducing catalyst in order to measure the NOx concentration of the exhaust gas. When NOx emissions increase, the air-fuel mixture supplied to the internal combustion engine is temporarily controlled to a rich air-fuel ratio so as to emit unburnt gas from the internal combustion engine. The unburnt gas reacts with NOx accumulated in the catalyst, thereby suppressing NOx emissions.
Since the above-mentioned NOx-concentration measuring apparatus cannot measure the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine, the realization of the above NOx control requires a separate air-fuel ratio measuring apparatus for obtaining an air-fuel ratio from the oxygen concentration of the exhaust. That is, in order to carry out the above NOx control, air-fuel ratio control must also be carried out simultaneously. To meet this end, both the NOx sensor and an oxygen concentration sensor (a so-called air-fuel ratio sensor) must be provided in the exhaust system of the internal combustion engine.
In view of the foregoing, an object of the present invention is to provide an NOx sensor control circuit unit which enables an existing NOx sensor to serve not only as an NOx sensor but also as an oxygen concentration sensor by attachment to the existing NOx sensor and which can thus simplify the configuration of a detection system for detecting NOx concentration and oxygen concentration (or air-fuel ratio). It is also an object of the present invention to provide an NOx sensor system including the control circuit unit.
The above objects of the present invention are achieved by providing an NOx sensor control circuit unit (hereinafter also referred to as a control circuit unit) adapted for use by connection to an NOx sensor. The NOx sensor for connection to the control circuit unit of the present invention comprises:
First processing chamber: The first processing chamber is an internal chamber of the NOx sensor and is isolated from the surrounding atmosphere. A measurement gas is introduced into the first processing chamber via a first diffusion-controlling passage.
Second processing chamber: The second processing chamber is an internal chamber of the NOx sensor and is isolated from the surrounding atmosphere. A gas contained in the first processing chamber is introduced into the second processing chamber via a second diffusion-controlling passage.
Oxygen concentration detection element: The oxygen concentration detection element is formed of an oxygen-ion conductive solid electrolyte, sandwiched between porous electrodes, and measures the oxygen concentration of gas in the first processing chamber.
First pump element: The first pump element is formed of an oxygen-ion conductive solid electrolyte sandwiched between porous electrodes, and pumps out oxygen from the first processing chamber.
Second pump element: The second pump element is formed of an oxygen-ion conductive solid electrolyte sandwiched between porous electrodes, and pumps out oxygen from the second processing chamber.
Heating element: The heating element heats the first pump element, the oxygen concentration detection element, and the second pump element.
The control circuit unit of the present invention comprises:
First pump element control circuit: The first pump element control circuit controls voltage applied to the first pump element to thereby control the partial pressure of oxygen in the first processing chamber such that an output voltage from the oxygen concentration detection element becomes substantially constant.
First pump current detection circuit: The first pump current detection circuit detects current flowing through the first pump element (first pump current) and outputs a detection signal indicative of the detected current (hereinafter referred to as a first pump current detection signal).
Second pump element control circuit: The second pump element control circuit applies a constant voltage to the second pump element in a direction so as to pump out oxygen from the second processing chamber.
Second pump current detection circuit: The second pump current detection circuit detects current flowing through the second pump element (second pump current) and outputs a detection signal indicative of the detected current (hereinafter referred to as a second pump current detection signal).
Heating control circuit: The heating control circuit controls heating performed by the heating element.
The first pump current detection signal is used to determine the oxygen concentration of the measurement gas. The first pump current detection signal and the second pump current detection signal are used to determine the NOx concentration of the measurement gas.
An NOx sensor system of the present invention comprises the above NOx sensor and the above control circuit unit connected to the NOx sensor.
According to the above NOx sensor control circuit unit or NOx sensor system, by employing the first pump current detection circuit and the second pump current detection circuit, the first pump current detection signal can be used to determine the oxygen concentration of the measurement gas, and the first pump current detection signal and the second pump current detection signal can be used to determine the NOx concentration of the measurement gas. Accordingly, by connection to an existing NOx sensor, the NOx sensor control circuit unit enables the existing NOx sensor to serve not only as an NOx sensor but also as an oxygen concentration sensor, and thus simplifies the configuration of a detection system for detecting the NOx concentration and the oxygen concentration (air-fuel ratio).
Studies conducted by the inventors of the present invention have revealed the following. Variations in the oxygen concentration of a measurement gas introduced into the first processing chamber influence the NOx concentration dependence of the second pump current. Thus, a conventional method for obtaining the NOx concentration from only the second pump current fails to accurately determine the NOx concentration. By contrast, the NOx sensor control circuit unit or the NOx sensor system of the present invention determines the NOx concentration based on both the detection signal indicative of the first pump current, which reflects the oxygen concentration of the measurement gas, and the second pump current detection signal, thus enabling highly accurate measurement.
In this case, the first pump element control circuit can also be considered to control the voltage applied to the first pump element such that the oxygen concentration of the gas introduced from the first processing chamber to the second processing chamber via the second gas passage becomes substantially constant.
The above control circuit unit may further comprise integration means for integrating the first pump element control circuit, the first pump current detection circuit, the second pump element control circuit, and the second pump current detection circuit. The integration means makes the control circuit unit more compact and facilitates its connection to the NOx sensor.
The above control circuit unit may further comprise a microprocessor which serves at least as heating control instruction means for instructing the heating control circuit to control heating performed by the heating element such that the temperature of the first pump element, that of the oxygen concentration detection element, and that of the second pump element approach a target temperature. Thus, by connecting the control circuit unit to the NOx sensor, the temperature of the elements can be controlled. Furthermore, the control circuit unit including the heating control instruction means can be made more compact.
The above control circuit unit may further comprise an A/D converter circuit for converting into digital signals the first pump current detection signal output from the first pump current detection circuit and the second pump current detection signal output from the second pump current detection circuit. Thus, the control circuit unit can directly output, as needed, the first pump current detection signal and the second pump current detection signal in the form of digital signals for enabling digital processing of the signals by a microprocessor or a like device.
In the case where the control circuit unit further comprises a microprocessor, the microprocessor may serve as oxygen concentration information generation means for generating information regarding the oxygen concentration of the measurement gas based on the first pump current detection signal which has undergone A/D conversion by the A/D converter circuit. The microprocessor may also serve as NOx concentration information generation means for generating information regarding the NOx concentration of the measurement gas based on the first pump current detection signal and the second pump current detection signal which have undergone A/D conversion by the A/D converter circuit. Thus, the oxygen concentration information and the NOx concentration information can be obtained from the control circuit unit.
The above control circuit unit may further comprise a D/A converter circuit for converting to an analog signal a digital signal related to at least any of oxygen concentration information, NOx concentration information, air-fuel ratio information generated on the basis of oxygen concentration information, and excess-oxygen ratio information generated on the basis of oxygen concentration information, among digital signals output from the above microprocessor, and for outputting a converted analog signal. Thus, these kinds of information can be made available in the form of analog signals, which can be used more easily as control signals for internal combustion engines such as automobile engines. The control circuit unit may further comprise a display device for displaying at least any of the oxygen concentration of the measurement gas, the NOx concentration of the measurement gas, air-fuel ratio, and excess-oxygen ratio on the basis of the digital signals. Thus, a user can visually grasp such information.
When the above control circuit unit is connected to the NOx sensor further comprising a temperature detection section for detecting the temperature of at least any of the first pump element, the oxygen concentration detection element, the second pump element and the heating element, the above microprocessor may serve as means for performing temperature correction for information regarding the concentration of an object component. The microprocessor generates oxygen concentration information corrected for temperature and NOx concentration information corrected for temperature (oxygen concentration information and NOx concentration information are hereinafter referred to generically as xe2x80x9cobject component concentration informationxe2x80x9d) on the basis of the temperature detected by the temperature detection section, the first pump current detection signal, and the second pump current detection signal.
Thus, even when the temperature of, for example, the oxygen concentration detection element, temporarily varies due to an abrupt variation of exhaust gas temperature, information regarding the concentration of an object component is generated in the form of concentration information corrected for temperature, thereby maintaining a high degree of detection accuracy. In this case, the temperature of the oxygen concentration detection element may be measured using a separate temperature sensor, such as a thermistor or a thermocouple. However, a solid electrolyte, which constitutes the elements, is characterized in that its internal resistance varies with temperature. Consequently, this feature may be utilized for measuring temperature, thereby yielding an advantage that there is no need for providing a separate temperature sensor, along with simplifying the measuring system. In this case, the oxygen concentration detection element serves as the temperature detection section, and the above control circuit unit may further comprise an internal-resistance measurement control circuit for measuring the internal resistance of the oxygen concentration detection element. In the case where the control circuit unit includes a microprocessor, on the basis of the measured internal resistance, the heating control instruction means implemented by the microprocessor instructs the heating control circuit to control heating performed by the heating element such that the first pump element, the oxygen concentration detection element and the second pump element are heated to a target temperature.
The above internal-resistance measurement control circuit may comprise an internal-resistance detection current application circuit for applying a constant internal-resistance detection current to the oxygen concentration detection element so as to measure the internal resistance of the oxygen concentration detection element. In particular, the internal resistance of the oxygen concentration detection element is simply obtained from the voltage that is developed across the oxygen concentration detection element. In this case, the microprocessor may serve as internal-resistance information detection means for detecting the voltage that is developed across the oxygen concentration detection element when a constant internal-resistance detection current is passed through the oxygen concentration detection element (this voltage is hereinafter referred to as a xe2x80x9cresistance detection voltagexe2x80x9d), as information regarding the internal resistance of the oxygen concentration detection element.
The above internal-resistance measurement control circuit may further comprise a modification current application circuit for applying a modification current to the oxygen concentration detection element in a direction opposite that of the internal-resistance detection current, after applying the internal-resistance detection current to the oxygen concentration detection element. When the internal-resistance detection current is applied to the oxygen concentration detection element, oxygen is transported within the oxygen concentration detection element in a direction opposite that of the current. This causes a variation in the oxygen concentration as measured at the opposite sides of the oxygen concentration detection element. As a result, when the NOx sensor resumes measuring the object component concentration, the variation of the oxygen concentration may become an error factor and thus impair accuracy in measuring the object component concentration. When the internal resistance of the oxygen concentration detection element is high, oxygen ions have difficulty moving through the oxygen concentration detection element, potentially causing polarization in association with the application of current. Thus, the modification current application means applies a modification current to the oxygen concentration detection element in a direction opposite that of the internal-resistance detection current after the internal-resistance detection current is applied to the oxygen concentration detection element. As a result of applying the modification current, oxygen is transported in reverse direction with respect to the above-mentioned transportation of oxygen. Thus, the varied oxygen concentration approaches a level as measured before the internal resistance is measured, thereby improving accuracy in resumed measurement of the object component concentration as well as canceling the polarized state of the oxygen concentration detection element. In this case, the magnitude and period of application of the modification current may be set such that the application of the modification current causes the reverse transportation of oxygen in an amount substantially equal to that transported when the internal-resistance detection current is applied. For example, a modification current substantially as large as the internal-resistance detection current may be applied for a period of time substantially equal to the period of time that the internal-resistance detection current is applied.
The above control circuit unit or NOx sensor system may further comprise a standard characteristics information storage section for storing information regarding predetermined standard characteristics (hereinafter referred to as xe2x80x9cstandard characteristics informationxe2x80x9d) representing correlations among the first pump current value, the second pump current value and the NOx concentration of the measurement gas; and a correction data storage element for storing correction data for making the previously measured characteristics of the NOx sensor equal to the standard characteristics. These characteristics represent correlations among the first pump current value, the second pump current value and the NOx concentration of the measurement gas. When the control circuit unit includes a microprocessor, the NOx concentration information generation means implemented by the microprocessor may detect a signal indicative of the first pump current and a signal indicative of the second pump current, may correct the detected values on the basis of correction data, and may generate information regarding the NOx concentration of the measurement gas using standard characteristics information.
Accordingly, even when the NOx concentration of the same measurement gas is measured using different NOx sensors, any of the NOx sensors can provide an accurate measurement. This is because variations in measurement among the sensors are corrected by the correction data peculiar to the individual sensors. There is no need for each NOx sensor to store characteristics which represent correlations among the first pump current value, the second pump current value and the NOx concentration of the measurement gas. That is, storage may be limited to standard characteristics and correction data, so that the storage capacity needed for storing such data is relatively small.